Silicon-on-insulator (SOI) is gaining interest as a material system for ultra-compact integrated photonic circuits. An advantage of this material system is the high refractive index contrast between the silicon and the adjacent oxide or air, enabling small bend radii and dense integration. Furthermore, as silicon is the standard material that is used for microelectronic integrated circuits, this opens the door for photonics to use the standard, mature and commercially available technology used in microelectronic fabs. SOI technology opens the possibility for integration of photonic and microelectronic devices on the same wafer.
Optical functionalities can be realized on SOI substrates by etching or writing a pattern in the functional silicon layer. This can for example be done with electron beam lithography, which is however a serial fabrication technique and therefore unattractive for mass fabrication. When using optical lithography for defining the patterns, variations of the critical dimensions are inevitable. These variations can be caused by wafer non-uniformity or by non-uniformity within one chip and can affect the behavior or the properties of the optical component. For example, ring resonators are highly sensitive to fabrication errors. A fabrication error of 1 nm typically leads to a 1 nm shift of the resonance wavelength. In practice the resonance wavelength shift can exceed 1 nm, which is unacceptable for many applications. Even the fabrication of identical devices on one wafer is difficult due to wafer scale variations. This problem may be solved by altering the devices after fabrication, for example by tuning or by trimming. Tuning leads to a time-dependent and reversible change of the optical device or component. Trimming results in an irreversible static change of the optical device or component.
Tuning of silicon-based optical components can for example be done thermally, by heating the entire chip comprising the optical components or by providing electrical heaters close to an area of interest. However, if one envisages a plurality of components, e.g. resonators, on a single chip, thermal tuning with electrical heaters requires the incorporation of a heater for every resonator, which may lead to high power consumption and a high device complexity. Another tuning method that may be used is carrier injection or depletion, requiring local doping of the silicon. For providing small modifications after fabrication, trimming techniques may be preferred instead of tuning techniques.
The resonant wavelength of a ring resonator is proportional to its optical path length, being the product of the physical path length and the effective index of refraction. Post-fabrication trimming of the resonance wavelength can therefore be obtained by trimming or changing this effective index of refraction. An increase in effective index causes a red shift (i.e. a shift towards a longer wavelength) of the resonance wavelength, while a decrease in effective index causes a blue shift (i.e. a shift towards a shorter wavelength) of the resonance wavelength.
In several low to medium index contrast material systems, trimming of the effective index of refraction was demonstrated based on direct UV irradiation or electron beam irradiation of the core material, such as silica glass, SiN or SiON. These core materials can be compacted when exposed to UV radiation or electron beam radiation, the compaction resulting in an increase in refractive index of the core material, and thus an increase of the effective index of refraction. As the compacted core material contains most of the optical mode, relatively large shifts in effective indices and thus in resonance wavelength can be obtained. For example, with SiN as a core material, more than 10 nm of resonance wavelength shift can be obtained for ring resonators operating at 1550 nm.
In high index contrast material systems, i.e. material systems with a large difference in refractive index (e.g. larger than 1) between a core material and a cladding material, such as for example SOI, the core material, such as for example silicon, is often not susceptible to compaction by either UV irradiation or electron beam irradiation.